Magnetic induction impedance measurements represent an non-invasive and contactless examining technique for examining an electrically conductive object. In particular, this technique is suitable for determining a parameter of the object, for example, a physiological characteristic of a person.
Conventionally, a coil arrangement of a magnetic induction impedance measurement apparatus comprises an excitation coil and one or more detection coils. The excitation coil is configured for generating a magnetic excitation field and the one or more detections coils are configured for detecting a magnetic response field generated in response to the magnetic excitation field inducing eddy currents in the object.
In operation of the magnetic induction impedance measurement apparatus, a time-varying current is induced in the excitation coil such that the excitation coil generates the magnetic excitation field which penetrates through the object to be examined and accordingly induces the eddy currents within the object. The magnitude of the eddy currents is based on the magnetic flux density of the magnetic excitation field and the conductivity of the object. The magnetic response field is generated by the flow of the eddy currents in the object and is detected by the one or more detection coils in that a current or a voltage is induced in the one or more detection coils. The induced current or voltage is a measure for the electrical conductivity of the object and depends on the conductivity and the geometry of the object and the geometry of the excitation coil and of the one or more detection coils.
Usually, the excitation coil is located in the vicinity of the one or more detection coils such that the magnetic excitation field is present in the one or more detection coils and accordingly causes an additional current or an additional voltage to be induced in the one or more detection coils. Further, a field strength of the magnetic excitation field in the one or more detection coils is orders of magnitude higher compared to a field strength of the magnetic response field in the one or more detection coils, thereby leading to a poor signal to noise ratio for the detection of the magnetic response field.
When determining the parameter of the object, a signal change which is caused from the parameter of the object in the one or more detection coils may comprise a low value in comparison to a signal value of an average signal caused by the object itself in the one or more detection coils. Consequently, it may be difficult to accurately determine the parameter of the object owing to the presence of the magnetic excitation field in the one or more detection coils and/or the low signal change value of the signal change caused by the parameter of the object.
It is known that the magnetic excitation field in the one or more detection coils may be compensated, in order to increase the accuracy of the determination of the parameter of the object.
WO 2010/052609 A1 describes a coil arrangement for a magnetic induction tomography system. The coil arrangement comprises at least one excitation coil and at least one detection coil. A field strength of the magnetic excitation field in the at least one detection coil is fully compensated or cancelled such that a signal caused by the magnetic response field in the at least one detection coil is usable for calculating a conductivity distribution of the object to be examined and for visualizing the conductivity distribution of the object.
EP 1 926 424 A2 describes a system and a method for inductively measuring a bio-impedance of a conductive tissue. The system comprises an excitation coil and a detection coil with an axis of the detection coil being oriented substantially perpendicular to flux lines of the magnetic excitation field passing through the detection coil. Further, the system comprises a shimming coil configured for generating a magnetic shimming field such that the magnetic excitation field in the detection coil is cancelled.
There may be still a need for determining a parameter of an object with a high accuracy.